There is continuous demand for higher efficiency and higher horsepower in aircraft internal combustion engines. Making an engine more efficient or more powerful is an exercise in compromise because a number of competing variables affect the performance of an engine.
For example, the efficiency and power output of a spark-ignited internal combustion engine are directly affected by both the timing of the spark and the compression characteristics in the combustion chamber. A poorly designed cylinder head may impede intake and exhaust gases from flowing optimally through the cylinder head, thereby causing flow losses and/or backpressure within the cylinder head and exhaust manifold. These variables, among others, can contribute to or denigrate the overall horsepower and efficiency of a spark-ignited internal combustion engine.
Conventional techniques for improving combustion in a cylinder head, and ultimately efficiency and horsepower output of the engine, include increasing temperature, pressure, and chemical residence time of the unburned fuel/air mixture, i.e., end gas. But these techniques often result in increasing the likelihood of engine knock, which may significantly degrade overall engine performance. Knock is the sound associated with a high-pressure wave front slamming against the internal surfaces of a combustion chamber in a spark-ignited internal combustion engine. The high-pressure wave results from unintended instantaneous ignition and combustion, i.e., autoignition, of the fuel/air mixture in advance of a spark. If autoignition occurs before the piston reaches top dead center (“TDC”), the pressure wave will actually push against the piston during the compression stroke thereby reducing the available energy for the power stroke, and ultimately, reduce engine power. Other conventional techniques to improve the efficiency and thoroughness of combustion in a cylinder head, such as increasing the compression ratio, off-center spark plug placement, and incorporation of slow-burn combustion chambers, can also lead to an increased likelihood of undesirable engine knock.
A faster-burning chamber can be useful in providing greater fuel efficiency as well as reducing engine knock. Characteristics of conventional faster burning chambers include the use of high-swirl intake ports and a rotational motion, i.e., swirl, of the charge due to off-cylinder-axis admission and the use of two or more spark plugs. The faster burning chambers induce small scale turbulence in the combustion chamber so that part of the piston head comes close to the cylinder head at TDC to thereby “squish” the charge in this region into a smaller, more highly compressed area of the combustion chamber and toward the spark plug tips. However, a faster burning chamber can impede the operation of other elements of the cylinder head.
One conventional method for producing a faster burning chamber is increasing the fuel/air mixture ratio, i.e., fuel enrichment, above the stoichiometric requirements for combustion. While fuel enrichment provides a cooler running engine in that expansion or evaporation of excess fuel absorbs energy, fuel economy will suffer.
Another aspect of conventional cylinder heads that contribute to reduced efficiency and power output of a spark-ignited internal combustion engine is the method of airflow into, and out of, the cylinder head. Optimal air intake and exhaust gas flow into and out of a cylinder head depends, among other things, on whether the intake and/or exhaust ports are sized correctly. If either the intake or exhaust ports are not sized to accommodate the flow rates demanded by the engine, restricted airflow and decreased efficiency and power output of the engine will result. In a conventional cylinder head, the intake and exhaust ports are typically designed to be wide and circular in shape. One conventional technique to decrease the possibility of any unintended flow resistance is to increase the size of the circular intake and/or exhaust port. However, enlargement of intake and exhaust ports are not without cost or impact on performance.